A centrifugal separator of this kind comprises a rotor, which is rotatable around a rotation axis in a certain rotational direction, which rotor forms inside itself                an inlet chamber, in which a conduit for the supply of a mixture of the two liquid phases, which are to be separated, opens        a separation chamber communicating with the inlet chamber,        an outlet device for the discharge of the specific light liquid phase separated during operation the outlet device including an outlet passage, which is connected to a radial inner portion of the separation chamber, and        another outlet device for the discharge of the specific heavier liquid phase separated during operation this outlet device comprising an outlet channel formed in the rotor, which extends radially and has an inlet opening at its radial outer end located at a certain radial level in a radial outer portion of the separation chamber and at its radial inner end opens in an outlet chamber surrounding the rotation axis, in which the specific heavier liquid phase forms a rotating liquid body having a radially inwardly turned free liquid surface. The radial position of the free liquid surface, during operation takes a position at a level in balance with the pressure prevailing in the separation chamber at the inlet opening, and in which a discharge device is arranged. The discharge devices non-rotatable with the rotor and has at least one internal discharge channel, which extends radially and at it radial outer end has an inlet opening. At a radial inner end the discharge channel is connected to an outlet, at least a radial outer part of the discharge device, in which the inlet opening is located, being movable in a way such that the inlet opening can be put in a different radial position in the outlet chamber.        
The centrifugal separator further comprises means for the supply of a predetermined volume of the specific heavier liquid phase to the separation chamber, a first indicating means for indicating that the separation chamber during operation is filled up to a certain wanted level, means for keeping the separation chamber filled up to this radial level, and a second indicating means for indicating the radial position of the free liquid surface in the outlet chamber for the specific heavier liquid phase,
In order to achieve a good separation result in a centrifugal separator it is of great importance at which radial level the interface between a light and heavier liquid phase is formed during an operation in the separation chamber of the centrifugal rotor. The interface will take a position at such a radial level that equilibrium will occur between the two liquid columns of the two liquid phases.
In order to maintain the interface at a wanted level in centrifugal separators, in which both the specific light and the specific heavier liquid phase forms a free liquid surface at one outlet out of the separation chamber each. The outlet for the specific light phase out of the separation chamber has been provided with an overflow outlet in the shape of a level ring surrounding the rotation axis and the outlet for the heavier liquid phase and as well with an overflow outlet in the shape of a control ring surrounding the rotational axis.
Generally if an unsatisfactory separation result is obtained one wishes to adjust the radial position of the interface to stop the centrifugal separator in order to exchange the control ring to a different control ring having another radius for the overflow outlet. Often, it is not enough to stop the centrifugal separator once to change the control ring but this has to be done several times before a control ring having a radius for the overflow outlet which gives a satisfactory separation result is found. This constitutes a difficult and time consuming operation and if for instance the density of one of the liquid phases in the mixture varies this can cause repeated down times.
To be able to adjust the radial position of the interface without the need for the centrifugal separator to be stopped for the exchange of such a control ring it has been suggested that instead of controlling the radial position of the interface by means of the radius of the overflow outlet, one provide the outlet device for the discharge of the specific heavier liquid phase with an outlet chamber, which during operation is pressure connected to the separation chamber via an outlet channel, whereby an obtained free liquid surface in the outlet chamber will be determining for the radial level of the interface. According to this suggestion a stationary discharge device is arranged in the outlet chamber, which has an internal discharge channel, which extends radially and in its radial outer end has an inlet opening and in its radial inner end is connected to an outlet, the inlet opening during operation being located radially outside the free liquid surface. The radial position of the free liquid surface is adjusted by means of a valve arranged in the outlet, which gives a variable counter pressure in the outlet, which so influences the free liquid surface in the discharge chamber that the higher the counter pressure is the bigger the radial distance between the free liquid surface and the inlet opening is. Thus, the counter pressure set in the outlet controls the radial position of the interface.
Naturally, the radial position of the interface can be controlled in a corresponding manner by adjusting the counter pressure in the outlet for the specific light liquid phase.
Whether the radial position of the interface is regulated by the exchange of the control ring or by adjusting the counter pressure in the outlet of the one or the other liquid phase you have no acceptable control of the radial level at which the interface is located. This means that a small change of the condition of the operation might have a great influence on the separation result. The control by means of adjusting the counter pressure according to the method described above furthermore results in an unacceptable generation of heat in the discharge of the chamber as a result of the stationary discharge device partly being immersed in the rotating liquid body in the outlet chamber.
The object of the present invention is to provide a simple method for adjusting the above-described interface to a wanted radial level without the need for the centrifugal separator to be stopped and disassembled.